Vapor generators operating on the balanced draft principle have long been known. In such units a forced draft fan operates to impart air for combustion to the furnace chamber while an induced draft fan operates to extract the generated combustion gas from the unit for ultimate discharge from the stack. According to common practice the forced draft fan is operated in response to the demand for combustion air as dictated by the fuel firing system controls. The induced draft fan, on the other hand, is controlled to provide a slightly negative (e.g. 0.5 inches water) fluid pressure in the furnace cavity thereby preventing the possibility of leakage of combustion gases from the furnace chamber to the exterior of the unit.
As the size of vapor generators has increased, the size and operating capacities of the forced draft and induced draft fans necessary to maintain balanced furnace pressures have increased concomitantly. Due to this fact, conditions have been created within the furnace cavity of large capacity vapor generators during operational excursions that have caused excessive negative fluid pressures therein to the extent that there have resulted furnace implosions of such magnitude as to cause serious damage to the unit.
Among the suggestions made with a view toward protecting against such implosions has been to construct vapor generators of greater structural strength thereby enabling the unit to withstand greater negative pressures. Such solution obviously increases significantly the fabrication cost of a vapor generator. Another suggestion would effect the rapid closure of the induced draft fan damper to prevent the extraction of combustion gas from the furnace while simultaneously admitting air under pressure through the forced draft fan. In addition to adding significantly to the cost of the unit such solution is of questionable effectiveness due to the inability to close the dampers with sufficient speed to prevent an implosion.
It is to a more effective solution to the above desdcribed problem, therefore, that the present invention is directed.